The task of housing is primarily to protect the electronics inside against harmful environmental influences. It is therefore particularly important to know possible dangers in advance and to take them into account in the development.
The degree of protection of housing is classified according to DIN EN 60529 with an IP code. This gives information about the influences under which the housing can be used without losing the protective function. The IP code consists of a two-digit number, with the first digit for the contact and foreign body protection and the second digit for the water protection.
Some conventional methods for sealing housings are the foaming of polyurethane seals, the use of chloroprene gaskets and the sealing with sealants. A proven and effective method in the automotive industry is the encapsulation of the electronics with epoxy resin. This protects the electronics not only against dirt and water but also against shocks and vibrations.
Outdoor enclosures use pressure balance elements that allow controlled breathing of the enclosure. These same pressure differences in the housing, which arise in the event of temperature fluctuations and may lead to failure of the housing seal. The protection class of a standard housing refers exclusively to the unprocessed condition. For subsequent processing, the tightness of the house must be rechecked.
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In addition to water and dirt, electromagnetic waves are also critical for the electronics. These are bound to the current flow. A distinction is made between desired electromagnetic waves, as in a mobile phone, and undesirably caused waves, as in a transformer. Developers have to deal with unwanted electromagnetic waves. For the CE conformity, the electromagnetic compatibility (EMC) of electrical equipment is mandatory. The European EMC directive 2014/30 / EU stipulates that electrical equipment must operate satisfactorily in an electromagnetic environment without generating interference to other stuff.
Electrical filters, sophisticated board layouts and local shielding of components make the system EMC-friendly for the most part. An EMC housing provides additional shielding. These housings have an electrically conductive surface; columns are sealed with special, electrically conductive seals. Embedded systems must also be sufficiently shielded so that foreign jammers cannot influence them.
Thermal design of embedded systems
When selecting or designing housing for an embedded system, thermal management plays a particularly important role. For years, the systems are usually invisible to the operator in difficult conditions. Only a thermally correctly designed housing ensures a long-term and fail-safe functioning of the sensitive electronics. For applications with waste heat, the housing manufacturers offer solutions with integrated heat sinks.
When selecting such a housing, the overall thermal resistance of the electronics plays an important role. The thermal resistance, also called Rth value, is given in Kelvin / Watt (K / W) and gives information about how many Kelvin temperature difference is needed to dissipate 1 W of power. For a rough rollover of the Rth value, the following formula is available:
For the detailed calculation of the Rth value, the thermal resistances in the heating chain, such as the thermal resistance of the semiconductor housing and the heat conducting materials, must be determined and taken into account. Using the thermal resistance determined, developers can use the heat diagram of the housing to check whether the housing is thermally suitable for the planned application. The Rth value of the housing must be smaller than the determined total thermal resistance of the heating chain. Because the smaller the Rth value, the more heat can be dissipated.
Whether a system is actively or passively cooled depends on the location and the power to be dissipated. For example, if the electronics are to operate in a dusty and damp environment in the future, passive cooling will be better suited than an active one. Passive cooling means that no active parts such as fans are required for cooling. The heat is transferred to a heat sink, which transfers the heat via the cooling fins to the environment. Using cooling fins, an increase in surface area is achieved, whereby a larger amount of heat, based on natural convection, enters the environment. Passively cooled systems have the advantage that they are quiet, low-maintenance and better protected against water and dust due to the missing fans.
At high power losses and an unfavorable installation position, a passively cooled system quickly reaches the thermal limits. In such cases, active cooling with fans is the right choice. Fan motors form with the appropriate heat sinks a fan unit, which is ideal for cooling tasks with high power dissipation. The disadvantage is that fans must be protected in a dusty environment with filter elements and speed and temperature monitoring need.
The right connection to the heat sink
Even though housing has been designed and manufactured specially for the geometry of the electronics, it is important to worry about the proper contacting of the heat generating components. Each system is to be viewed and evaluated individually. The challenge is to extract the heat at maximum power from the source and transfer it directly, with little loss, to the heat sink, in this case, the heat sink.
But that’s not always easy to do. Sometimes the board geometry does not allow a direct connection to the housing or the heat concentrates on a too small area. In such cases, heat pipes are used in conjunction with copper heat conducting sheets. Heat pipes, called heat pipes, are heat conducting elements made of copper with a special structure, which have very high heat conduction properties.
Surfaces for thermal bonding must be level and clean. To exclude air pockets and to keep the heat transfer resistance as low as possible, despite the flat surface Wärmeleitfolien or pastes are used.
Computer-aided heat simulation is a common method for thermally analyzing systems, for example. Already in the early development phase, it gives developers insights into whether the developed cooling concept adequately fulfills the purpose. 3D data and component parameters can be used to simulate and analyze different realistic situations. Due to the visual representation of the simulation results, problem areas become quickly apparent and can be optimized cost-effectively at an early stage. Some housing manufacturers offer computer-aided heat simulation as a service. Also, the electromagnetic behavior can be simulated computer-aided.